Effects of nanoparticles size and concentration and laser power on nonlinear optical properties of Au and Au–CdSe nanocrystals

Applied Surface Science 353 (2015) 112–117

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Effects of nanoparticles size and concentration and laser power on nonlinear optical properties of Au and Au–CdSe nanocrystals Abeer Salah, A. Mansour, M.B. Mohamed, I.M. Azzouz ∗ , S. Elnaby, Y. Badr National Institute of Laser Enhanced Sciences, Cairo University, Giza 12613, Egypt

a r t i c l e

i n f o

Article history: Received 10 October 2013 Received in revised form 13 June 2015 Accepted 14 June 2015 Available online 20 June 2015 Keywords: Au plasmon CdSe QDs Nonlinear optical absorption

a b s t r a c t Au and Au–CdSe nanoparticles (NPs) have been synthesized by organometallic pyrolysis method. Nanocrystals (NCs) structure was confirmed using high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD). Nonlinear optical absorption is investigated by Z-scan technique using nanosecond laser pulses of second harmonic Nd:YAG. Intensity-dependence of nonlinear absorption on both nano-size and concentrations is reported. These are interesting findings which can be used to fabricate optical limiting and optical switching devices from NPs and hybrid systems. © 2015 Elsevier B.V. All rights reserved.

1. Introduction In recent years, nonlinear optical properties of nano particles have attracted many interests due to their various potential applications in optoelectronic and biological fields such as, optical limiting [1–3], multi-photon imaging [4], all optical switching [5,6] cell-biology and cancer-therapy [7–9]. As noble metals reduced in size to tens of nanometers, a new and very strong visible absorption band is observed resulting from the collective oscillation of conduction band electrons known as surface plasmon. Also, semiconductor quantum dots (QDs) show new properties due to confinement of electronic motion to a length scale comparable to, or less than, that characterizing the electronic motion in bulk state (Bohr radius, few nanometers) [10]. Combination of nano metals and semiconductors has acquire particular interest for basic sciences and technological applications [11,12,8]. For instance, it was found that QDs in vicinity of metallic NPs enhances, or quenches, their absorbance and fluorescence depending on both their intermolecular distance and NPs size [13–15]. Electrons transfer was also reported from conduction band to metallic NPs yielding enhancement in the charge separation and reduction in electron–hole pair recombination [16,17]. Enhancement was also reported in photo-electrochemistry properties of NPs/QDs bi-layers system [18].

∗ Corresponding author. E-mail address: [email protected] (I.M. Azzouz). http://dx.doi.org/10.1016/j.apsusc.2015.06.060 0169-4332/© 2015 Elsevier B.V. All rights reserved.

The unique properties stem from exciton–plasmon interactions allow for optoelectronic devices such as light emitting diodes [19] and solar cells [20] and biological applications [7–9,21,22]. This hybrid semiconductor–metal NPs not only combine their unique properties but also generate collective new phenomena based on the intra-particles interaction at the interface between metallic NPs and semiconductor QDs. Optical nonlinear properties of nano-structured particles have been proved and characterized by many techniques. Z-scan is one of the most popular spectroscopic techniques to estimate third-order nonlinearities [23–26]. This technique has many configurations [26] such as EZ-scan, white light Z-scan, two color beam. It based on transformation of phase distortion to amplitude distortion of propagating beam through a nonlinear medium. The technique measures transitivity of a medium, through a finite aperture in the far field, at different positions (Z) from a focal plane. Closed aperture Z-scan (aperture in front of detector) is used to measure nonlinear refractive index of a medium whereas nonlinear absorption coefficient is measured using open aperture Z-scan (no aperture in front of detector). Z-scan is the most widely used technique due its simplicity and accuracy in measuring nonlinearity magnitude and sign [25]. In this work, Au NPs and nano-composite of Au–CdSe, at different NPs sizes and concentrations, were prepared and characterized by different methods. Nonlinear properties of the samples were confirmed using open aperture Z-scan technique. Dependence of nonlinear optical properties on laser excitation energy is detected for different NPs’ size and concentrations.

A. Salah et al. / Applied Surface Science 353 (2015) 112–117

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Fig. 1. Linear absorption of Au NPs and Au–CdSe at different sizes of CdSe QDs.

2. Methods and techniques

Fig. 2. Linear absorption of Au–CdSe at different concentrations of 3.54 nm CdSe.

Au NPs have been prepared by pyrolysis of organometallic method. Solution of Au:HCl (10 ␮L) was injected into a mixture of oleic acid and oleyl amine (OA) in microwave for 30 s. The solution color turns to be finite pink. Formation of Au NPs has been confirmed by absorption spectra and TEM images. Au–CdSe nanocomposites had been prepared as given in Ref. [27]. Au NPs solution was heated up to 120 ◦ C in a three necked flask then CdSe precursor (CdO soluble in oleic acid) was added. Se–TOP solution was injected quickly at 180 ◦ C. Samples have been withdrawn at different time intervals yielding different sizes of hybrid nanostructure. HRTEM images were taken using transmission electron microscope, JEM JEOL-2010 model, with an accelerating voltage of 200 kV. Methanol was added to separate the samples from the reaction mixture and bi-product. The centrifuged precipitant was re-dispersed in toluene before imaging. Linear absorption was investigated using Perkin Elmer Lambda 35 spectrometer in the range from 400 to 800 nm.

Fig. 3. HRTEM of Au–CdSe composite.

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A. Salah et al. / Applied Surface Science 353 (2015) 112–117

T (z) =

∞ m  −q0 /(1 + (z 2 /z 2 )) 0

(m + 1)3/2

m=0

(1)

where q0 = ˇI0 Leff for |ˇI0 Leff |<1, ˇ is the nonlinear absorption coefficient, Leff = (1 − e−˛o L )/˛o is the effective interaction length with a sample of thickness L, and ˛0 is the linear absorption coefficient. ˇ can be determined by fitting this expression with experimental results. Im (3) obtain in terms of the following relation [12]: Im (3) =

n20 ε0 c 2 42

ˇ

(2)

Samples experiences maximum pump intensity at focus. Saturable absorber (SA) and reverse saturable absorber (RSA) are nonlinear optical properties may be studied using open aperture Z scan technique. They result as an increase (peak) or a decrease (valley) in transmittance curve at focus. 3. Results and discussion Room temperature linear absorption spectra are recorded in Fig. 1 for Au NPs and hybrid Au–CdSe nano-composite at different sizes of CdSe QDs. The spectra display the features of CdSe QDs excitonic and surface plasmon (SP) Au NPs. Peaks at 538, 576, 590, 600

(1 1 1 )

Au (2 0 0 )

Intensity (a.u)

Nonlinear absorption was determined by Z-scan technique using 6 ns pulse duration of 532 nm Q-switched of 2nd harmonic Nd:YAG laser (Continuum). Repetition rate was 1 Hz to reduce possible effect of thermal accumulative. The working wavelength is very close to the spectral location of maximum absorption in Au–CdSe. Laser energy was varied from 50 ␮J to 1500 ␮J using different neutral density filters. These energy values were below threshold breakdown of the investigated samples. Care was taken to prevent material optical breakdown and irreversible changes produced by laser radiation. The experiment was carried out several times at the same point on the sample to ensure there is no structural change in materials. The sample automatically moved along propagation direction of focused laser using integrated long travel stage of 150 mm travel (LTS 150 M THORLABS). A lens of 150 mm focal length used for focusing laser beam to a spot of waist radius ω0 = 58 ␮m. Beam radius at different positions from focused spot was calculated according to ω2 (z) = ω02 (z)(1 + z 2 /z02 ), where zR = ω02 / is Rayleigh length = 20 mm. This length is much larger than the thickness of substrate and film together. To reduce influence of laser power fluctuation, the energy of transmitted beam and reference beam and their ratio were measured by a dual channel optical power meter (PM 320 E Thorlabs) using two photodiode detectors (S142C Thorlabs). The on-axis focused intensities ranged from 1.85 × 1012 to 2.36 × 1013 W m−2 . The normalized transmittance expressed as [24]:

(1 0 0 )

(0 0 2 )

(1 1 0 )

(2 2 0 )

(1 0 3 ) (1 1 2 )

(1 0 2 )

(1 0 0 ) (0 0 2 )

(1 1 0 )

(1 1 1 ) (1 0 2 )

(3 1 1 )

CdSe

(2 0 2 )

(2 0 0 ) (1 0 3 )

A u -C d S e

(1 1 2 )

(3 1 1 )

(2 2 0 )

(2 0 2 )

20

30

40

50

60

70

80

2θ ° Fig. 4. XRD patterns of: CdSe QDs, Au NPs and Au–CdSe nanocrystals.

and 613 nm are corresponding to CdSe QDs sizes of 3.37, 3.54, 3.91, 4.56 and 5.07 nm, respectively. Red shift is observed in absorption peak position as particle nano-size increased. The effect of concentration of NPs on linear absorption is presented in Fig. 2 at size 3.54 nm of CdSe QDs. The absorption bands peaked at same wavelength and obeying Beer’s law. The formation of NPs size and shape and crystalline phase has been confirmed by high resolution TEM (Fig. 3). In these images, Au NPs appear as a dark spot whereas CdSe QDs are not clearly visible due to lack of contrast. X-ray diffraction XRD has been used to examine crystal structure of the prepared NPs. The results are presented in Fig. 4 for Au NPs, CdSe QDs and Au–CdSe NPs. Crystal structure of the samples are proved by XRD patterns. For Au NPs samples, peaks are observed at 2 = 38.23◦ , 44.18◦ , 64.82◦ and 77.85◦ . These peaks attributed to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, respectively, of face cubic center Au crystal (JCPDS # 01-089-3697). The high intensity at 2 = 38.23◦ suggesting that (1 1 1) plane is the predominant orientation of Au NPs. For CdSe QDs, peaks observed at 2 = 21.83◦ , 25.51◦ , 34.89◦ , 42.52◦ , 45.86◦ , 49.97◦ and 55.75◦ are attributed to (1 0 0), (0 0 2), (1 0 2), (1 1 0), (1 0 3), (1 1 2) and (2 0 2) planes, respectively, of hexagonal CdSe crystal (JCPDS # 00-002-0330). Also, it is noticed that some reflections of ZB–CdSe overlap some reflections of wurtzite phase.

Fig. 5. XPS data of Au–CdSe nanocrystals.

A. Salah et al. / Applied Surface Science 353 (2015) 112–117

a

b

Au(O-Acid-O.Amine) 1.3 1.2

118µJ fit 118µJ 237µJ fit 237µJ 473µJ fit 473µJ 750µJ fit 750µJ 1496µJ fit 1496µJ

1.1 1 0.9 0.8 0.7 0.6

118µJ

1.4

-40

-20

0

20

40

fit 118µJ 237µJ fit 237µJ

1.3

376µJ fit 376µJ

1.2

473µJ fit 473µJ

1.1

750µJ fit 750µJ

1

944µJ fit 944µJ

0.9

1496µJ fit 1496µJ

0.8

0.5 -60

Au@CdSe at CdSe Size 3.37nm

1.5

Normalized Transmission (a.u.)

Normalized Transmission (a.u.)

1.4

115

-60

60

-40

-20

Z (mm) at 118 µJ

1.3

1.2

1.1 Au seed fit Au seed

1

Au:CdSe 3.37 nm fit Au:CdSe 3.37 nm

-40

-20

0

20

40

60

40

60

at 1496 µJ

1.4 1.35 1.3 1.25 1.2 1.15 1.1 1.05

Au seed fit Au-seed

1

Au:CdSe 3.37nm

0.95

Normalized Transmission (a.u.)

1.4

-60

20

at 237 µJ

1.5

Normalized Transmission (a.u.)

Normalized Transmission (a.u.)

c

0

Z (mm)

fit Au:CdSe 3.37nm

-60

-40

-20

0

20

40

60

Z (mm)

Z (mm)

1.1 1 0.9 0.8 0.7 0.6

Au Seed fit Au-Seed Au:CdSe 3.37nm

0.5

fit Au:CdSe 3.37nm

-60

-40

-20

0

20

40

60

Z(mm)

Fig. 6. (a) OA Z-scan transmission at different laser energies of Au NPs. (b) OA Z-scan transmission at different laser energies of Au–CdSe NCs (CdSe size = 3.37 nm). (c) Comparison between OA Z-scan transmissions of Au and Au–CdSe at I0 = 118 ␮J, 237 ␮J and 1496 ␮J.

XRD of Au–CdSe hybrid displays both Au and wurtzite CdSe patterns. Chemical compositions of Au–CdSe hybrid nanostructure were confirmed by XPS analysis (Fig. 5). As expected, the binding energies of Cd 3d5/2 , Se 3d and Au-4f electrons are observed at 405.6 eV of Cd (+II), 54.1 eV of Se (−II), and 87.8/84.2 eV of Au0 (4f5/2 /4f7/2 ). No significant difference in binding energy is observed with respect to Cd 3d and Se 3d in pure CdSe QDs. This indicates that no change in oxidation state or chemical environment of Cd or Se atoms in Au–CdSe hybrid NPs.

The effect of laser intensity on nonlinear absorption of nanocrystals has been studied. OA Z-scan measurements have been carried out, at different excitation intensities I0 (from 1.85 × 1012 to 23.6 × 1012 W m−2 ), for both Au NPs and Au–CdSe (CdSe size = 3.37 nm) presented by symbols in Fig. 6a and b, respectively and also in Fig. 6c, for easy comparison. The solid lines in these figures show the calculated transmittance values obtained by fitting experimental data with Eq. (1). Similar transmission behaviors are observed for both NPs. At low intensity, the normalized transmission shows a peak at the focus (z = 0) which is an indication for

Fig. 7. OA Z-scan of Au–CdSe at different sizes and at I0 = 118 ␮J.

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A. Salah et al. / Applied Surface Science 353 (2015) 112–117

Table 1 Nonlinear absorption parameters of CdSe:Au at different QDs sizes.

Table 2 Nonlinear absorption parameters for CdSe:Au at different concentration.

CdSe size (nm)

˛0 (cm−1 )

Leff (cm)

ˇ (−10−10 m/W)

Im (3) (−10−12 e.s.u.)

Conc. ratio of QDs size 3.54 nm

˛0 (cm−1 )

Leff (cm)

ˇ (−10−10 m/W)

Im (3) (−10−12 e.s.u.)

3.37 3.54 3.91 4.56 5.07

3.77 4.07 4.72 4.49 3.34

0.18 0.17 0.16 0.16 0.19

9.8 8.5 5.0 1.8 1.4

24 20 12 4.3 3.4

1/2.5 1/5 1/10 1/20 1/40 1/80

12.43 6.72 4.04 2.16 1.16 0.61

0.079 0.13 0.17 0.22 0.25 0.27

29 12.7 4.96 2.19 0.908 0.311

69.9 30.5 11.9 5.26 2.19 0.075

Nonlinear absorption β (nm/W)

0.0

Au NPs Au -CdSe -0.2

-0.4

5

10

15 12

2

20

25

Intensity x10 (W/m ) Fig. 8. OA Z-scan transmission of Au–CdSe at 0.31 GW/cm2 for different concentration of CdSe (3.54 nm).

Fig. 9. Power-dependence of nonlinear absorption coefficients of Au and Au–CdSe NPs (CdSe sizes = 3.37 nm).

SA nonlinear behavior. As the pump intensity increases, a central valley is observed within the peak shape. Further increase in I0 , the valley increases in depth accompanied with a gradual reduction in peak until complete valley shape is dominated (RSA behavior). This means that nano-crystal invert their nonlinear behavior from SA to RSA as the pump intensity increases. This behavior may be attributed to TPA process. The effect of nano-size of CdSe on nonlinear absorption of hybrid Au–CdSe has been investigated in Fig. 7. OA Z-scan measurements were carried out at constant laser energy (I0 = 118 ␮J). Figure show that, at this relatively low intensity, the transmission via NPs of different sizes, increases near the focused beam (z = 0). This is an indication for saturation behavior; absorption of ground state is higher than that of excited states (bleaching of Plasmon’s ground state). As the NPs size increases, the peak shape characterized the SA behavior decreases in height. In other words, SA decreased as QDs size increased. This decrease is clear in Table 1, where calculated values of nonlinear absorption coefficients are summarized together with different parameters used in the fitting process. The value of ˇ decreased in magnitude from 9.8 to 1.4 (10−10 m/W) as the QD size increased from 3.37 nm to 5.07 nm. This attributed to the decrease in absorption associated with the increase in QDs size. The effect of NPs concentrations on their nonlinear absorption behavior is examined in Fig. 8. OA Z-scan transmission at laser energy 195 ␮J (0.31 GW/cm2 ) were carried out for different concentrations of Au–CdSe at CdSe QDs size = 3.54 nm. The observed SA nonlinear behavior is found to increase as concentration of NPs increases. The variation of calculated nonlinear absorption coefficients of Au–CdSe on their concentration is displayed in Fig. 10 and summarized in Table 2. Fig. 9 shows the energy dependence of the calculated nonlinear absorption coefficients “ˇ” for Au NPs (circle symbols) and Au–CdSe NPs at CdSe sizes = 3.37 nm (square symbols). Similar trend is

observed for dependence of ˇ on laser energy. ˇ increases with a fast rate as laser intensity increase to 473 ␮J (∼7.5 × 1012 W m−2 ) then with a slower rate at higher intensities. Also, it is noticed that around this laser energy nonlinear behavior begin to turns over from SA to RSA for both Au NPs and Au–CdSe (=3.37 nm) (Fig. 6b). At low laser intensity, SA behavior disappears for bigger size of CdSe QDs (=5 nm) (see Fig. 7). Au NPs show higher values of ˇ. Larger size of CdSe NPs affecting ˇ at different energies of applied laser however smaller size do not show any variation on ˇ at lower energy laser. Fig. 10 presents the dependence of normalized transmittance on excitation energies at focus. Transmission was recorded at

Fig. 10. Energy-dependence of normalized transmittance at the focus for Au–CdSe.

A. Salah et al. / Applied Surface Science 353 (2015) 112–117

different energies through colloidal Au–CdSe NPs in a quartz cuvette of thickness 3 mm. Measurements were carried out for two NPs sizes (the smallest and the largest prepared NPs sizes). Larger sizes of QDs limit energy transmission at lower threshold. This was attributed to the departure of pump wavelength from the near resonant absorption as the size of the NPs increases. These results brings into focus the possibility of utilizing these NPs and NPs/QDs as smart materials for potential applications in the field of nonlinear optics such as optical limiting, sensor protection and all optical switching devices. 4. Conclusion Nanocrystals of Au and Au–CdSe NPs have been synthesized and characterized using high resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD). Nonlinear optical absorption was measured as a function of excitation energy. OA Z-scan technique was applied at near-resonant excitation using 532 nm of 6 ns Q-switched Nd–YAG laser. The measurements were carried out for different NPs sizes and concentrations. The location of SP energy and excitonic peak shifted from 538 nm to 613 nm as the NPs size increases. Increase was observed in nonlinear absorption coefficients and attributed to the resonance between surface plasmon of Au NPs and QDs exciton band. Nonlinear SA behavior was recorded on pumping samples with low power densities. Saturation absorption was reversed by increasing power densities. Such changeover in nonlinear coefficient’s sign was related to the interplay of plasmon bleach and optical limiting mechanism [28]. References [1] H. Pan, W. Chen, Y.P. Feng, W. Jia, J. Linb, Appl. Phys. Lett. (2006) 88. [2] S. Qu, C. Du, Y. Song, Y. Wang, Y. Gao, S. Liu, Y. Li, D. Zhu, Chem. Phys. Lett. 356 (3–4) (2002) 403–408.

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